Diatomite formations are unique due to a high oil content and porosity, while having such low permeability that the hydrocarbons have no natural flow path to a production location. In the case of one low permeability formation type, the very low permeability is a characteristic of the morphology of diatomite itself, where skeletal remains of ancient diatoms allow flow only through tiny micropores and openings caused by skeletal decrepitation. The naturally existing flow paths existing in a diatomite reservoir are usually much too small to support flow of fluid, let alone viscous heavy oil. Conventional heavy oil techniques such as conventional cyclic steaming or steam drive, both of which are well known, are not well suited for diatomite because of its extremely low relative permeability. The steam would merely bypass large portions of the diatomite reservoir and other formations. In such a low permeability reservoir, fluid can be injected successfully only after first fracturing the formation by injecting fluid at pressures exceeding the fracture pressure. A significant improvement in diatomite oil recovery technology would require a means to displace oil from the interior of the diatoms themselves. In addition, an improved flow path, or increased permeability, would be required to assist the flow of displaced oil from the reservoir interior to a production position, i.e., a wellbore.
The literature has seen many attempts aimed at recovering oil from diatomite formations. U.S. Pat. No. 4,167,470 teaches one method of recovering oil from diatomite in which a hydrocarbon solvent is contacted with diatomite ore from a mine in a six-stage extraction process. Solvent is recovered in a steam stripping apparatus. There are several problems in utilizing this solvent process in a cost effective operation. One major drawback is that the diatomite ore must be mined, carrying significant environmental and economic drawbacks, and the process is extremely complex and intensive. Furthermore, the process cannot be carried out in a manner utilizing equipment typical to oil field operations.
U.S. Pat. No. 4,828,031, assigned to the assignee to the present invention, is an improved method of recovering oil from diatomite formations. A solvent is injected into the diatomite and is followed with a surface active aqueous solution. The solution contains a diatomite/oil water wettability improving agent and surface tension lowering agent. The method may be enhanced by the injection of steam into the diatomite formation. No teaching is made, however, of the methods described herein for creating and enhancing a fracture flow path with controlled fracturing technique. U.S. Pat. No. 4,828,031 is useful, however, in the present case for a description of the general problems associated with production of oil from diatomite formations.
U S. Pat. No. 4,645,005 teaches a production technique for heavy oils, in unconsolidated reservoirs as opposed to diatomite. The formation may be fracture stimulated with steam prior to completion by conventional gravel pack. However, U.S. Pat. No. 4,645,005 fails to teach how fracture initiation and growth is controlled, and makes no teaching of dealing with the special considerations present with a very low permeability reservoir.
Methods of fracturing formations using bridge plugs and sandback techniques in combination with a pumped hydraulic fluid have been described. One such reference is in Hydraulic Fracturing, SPE Monograph Series Vol 2, by G. C. Howard et al., at pages 99-100.
It is apparent that an improved method of producing oil from low relative permeability formations such as diatomaceous formations is much desired.